The overall objective is the determination, within the next five years, of a small spherical virus structure at atomic resolution using primarily X-ray diffraction of single crystals. We have recently shown this virus to possess T quals 4 icosahedral symmetry; that is, there are 240 identical protein subunits in its coat (previous determinations were close to 180 subunits). Within the next few months we anticipate having a 22 angstrom unit resolution electron density map of Southern Bean Mosaic Virus (SBMV) which should characterize the spatial distribution of the morphological and structural units in this isometric, single-stranded, RNA - containing, plant virus. Within the next half year or less we should have an 11 angstrom unit resolution electron density map (based upon molecular replacement). Very soon afterwards, this map should be improved by a single isomorphous replacement derivative hopefully permitting us to see some of the secondary structural elements of both protein and nucleic acid. We have now shown that our present experimental set up is capable of collecting at least 5 angstrom units resolution data and thus anticipate having a 5 angstrom unit resolution map based upon molecular replacement and a single isomorphous replacement derivative possibly within one year from now. Eventually it will be necessary to extend our data to 3.0 angstrom unit resolution in order to attain our stated objectives. This should be possible with only very slight modification of our present experimental procedures. We anticipate that a detailed structural analysis of a virus will not only show us the nature of the protein-protein interactions of the coat, the protein-nucleic acid interactions in the core and the secondary structural elements of the nucleic acid, but will also be most informative of the relationship a virus plays to its host cell.